The invention relates to methods for the synthesis of ultrafine oxide particles or gels from an inorganic metal salt.
Ultrafine oxide powders, also referred to as sols when they are in a colloidal stable suspension, and gels are important building blocks for structural and functional materials, such as advanced ceramics and alloys for structural, thermal, optical, catalytic, separations, and electronic applications. Particle quality in terms of size, size distribution (monodispersity), chemical homogeneity/purity, and morphology (state of agglomeration and shape of primary particles) directly affects later-stage ceramic processing (such as sintering) as well as physical/chemical properties of the final products. Nanosized or nanostructured building block materials are strongly needed for fabricating so-called nanophase materials that have dramatically improved properties relative to the coarse-grained materials. Sol-gel processes produce coatings and films that exhibit unique properties suitable for various applications.
Currently, there is a strong need for more efficient and economical methods of production of high-quality inorganic particles. The primary technologies for synthesis of ultrafine particles are wet chemical synthesis, such as sol-gel processing, and gas-phase condensation. Gas-phase reactions typically require extreme conditions such as high vacuum and high temperature and give very slow particle production rates.
In sol-gel processes, a molecular precursor in a homogenous solution undergoes a succession of transformations: a) hydrolysis of the molecular precursor; b) polymerization via successive bimolecular additions of ions, forming oxo-, hydroxyl, or aqua bridges; c) condensation by dehydration; d) nucleation; and e) growth. Molecular precursors include a cation of a metal, in the form of an ionized inorganic species, such as a salt, or in the form of nonionized organometallic molecule, such as an alkoxide.
Alkoxide compounds have been used as the molecular precursors in most of the currently used or suggested sol-gel processes for synthesis of ceramic powders, films, coatings, and monoliths. However, alkoxides are too expensive for some applications. Moreover, the rapid reaction rate typically exhibited by alkoxides renders the reactions difficult to control. Further, the use of organic elements often results in organic contaminants that must be burned away at high temperatures. Typically, methods that utilize inorganic salts as precursors for wet chemical or solution synthesis include forced hydrolysis or hydrothermal processing, which usually requires relatively high temperatures of 100 to 200xc2x0 C. or higher to induce hydrolytic reactions, condensation and the resulting evolution of solid-phase particles. Nanosize hydrous zirconium oxide particles have been produced successfully by the forced hydrolysis although the process kinetics is relatively slow. Bleier, A. et al. xe2x80x9cNucleation and Growth of Uniform m-Zr02xe2x80x9d Mat. Res. Soc. Symp. Proc. 73:71-78 (1986).
Little work has been reported utilizing an inorganic metal salt and an organic solvent, such as an alcohol, together in materials synthesis and processing. For preparation of ceramic oxide powders, a xe2x80x9cliquid dryingxe2x80x9d process has been reported, in which an alcohol solvent was used as a hygroscopic liquid for the dehydration of an aqueous solution of a metal salt. The method involves spraying or atomizing the aqueous solution of a metal salt into the vortex of a swirling bath of hygroscopic liquid. The drying liquid rapidly removes water from the salt solution droplets while the remaining metal salts coprecipitate forming a powder. Jaeger, R. E. et al., xe2x80x9cPreparation of Ceramic Oxide Powders by Liquid Drying,xe2x80x9d Ceramic Bulletin 53(12):855-859 (1974); and O""Toole, M. P. et al. xe2x80x9cY203-Zr02 Powder Synthesis via Alcohol Dehydration of Aqueous Salt Solutions,xe2x80x9d Ceramic Bulletin 66(10): 1486-1489 (1987). The quality in terms of particle size monodispersity and particle shape uniformity of the powders produced in these methods is poor.
A process has been reported for production of hydrous oxide particles using a mixed aqueous/organic solvent and inorganic metal salt. However, the process is not a sol-gel process and the hydrous oxide particles are not nanosize. The process focuses only on ceramic powder production and the powder particle size was relatively large (from submicrons to a few microns). Moon, Y. -T. et al. xe2x80x9cPreparation of Monodisperse and Spherical Zirconia Powders by Heating of Alcohol-Aqueous Salt Solutions,xe2x80x9d J. Am. Ceram. Soc. 78(10):2690-2694 (1995).
Accordingly, what is needed is more efficient and economical methods of producing nanosized particles. What is needed is methods of producing sols and gels from inorganic metal salts at room temperature. What is also needed is methods of sol-gel processing using inorganic metal salts.
The present invention is directed to methods for sol-gel processing using inorganic metal salts.
The present invention also is directed to methods for producing nanosize sol particles from inorganic metal salts.
The present invention is further directed to methods for producing sols from inorganic metal salts at room temperature.
The present invention is also directed to particles, sols, and gels produced according to the methods described herein.
The present invention is further directed to articles made from sols and gels made according to the methods described herein.
The methods generally involve mixing together an inorganic metal salt, water, and a water miscible alcohol or other organic solvent, preferably at room temperature. A macromolecular dispersant material, such as hydroxypropyl cellulose (HPC), may optionally be added. The resulting homogenous solution is incubated at a temperature from about 20xc2x0 C. to 150xc2x0 C., preferably about 22xc2x0 C. to about 120xc2x0 C., for a period of time between about 1 minute to 72 hours, preferably about 1 to 48 hours. The mixture turns milky as the incubation proceeds. After incubation, the solution is optionally neutralized by titration with ammonia (NH4OH), for example, to prevent dissolution. Solids can be collected, such as by filtration or centrifugation, and optionally washed with deionized water.
Several parameters of the method can be manipulated, making the method highly tunable, and enabling production of sols and gels with various desired characteristics. For example, variables that can be tightly controlled and which control the product characteristics include the metal salt concentration (C), ratio of organic solvent to water (RH), temperature of incubation (T), time of incubation (t), and concentration of macromolecular dispersant (such as HPC).
The methods enable production of high quality sols and gels at lower temperatures than standard methods. The methods enable production of nanosize sols, having a diameter from about 10 nm to about 200 nm, preferably less than about 100 nm, from inorganic metal salts. The methods offer sol-gel processing from inorganic metal salts.